The left anterior descending coronary artery of anesthetized dog hearts will be alternately occluded or perfused with blood devoid of oxygen in order to differentiate effects of myocardial hypoxia from those of ischemia. The effects of myocardial hypoxia per se on the following parameters will be defined. 1) Myocardial contractility as measured by Walton-Brody strained gage arches sutured on the myocardium in the region perfused with hypoxic blood. 2) Shunting of various size microspheres through hypoxic myocardium. In addition, the relationship between coronary perfusion pressure and shunting of microspheres in hypoxic myocardium will be studied. 3) Transmural distribution of coronary blood flow during hypoxia and during recovery from ischemia and non-ischemic, myocardial hypoxia. 4) Overlapping of coronary perfusion fields. The inflow of arterial blood into hypoxic, non-ischemic myocardium will be studied by tracer techniques including radioactive microspheres and radioactive rubidium.